Almost all vertebrates enjoy a little bit of recess, even the ones that don't look like it. It's easy to imagine dolphins and panda bears having fun, spiking beach balls and rolling down hills. But crocodiles look all-business -- the deadly business of hunting and eating prey.

But new research proves crocodiles aren't too serious to let loose every once in a while. As the study by researchers at the University of Tennessee shows, surfing waves, playing ball, and taking piggyback rides are just of the few of the ways crocs like to have fun.

Animal psychologists divide play into three different types: locomotor play, play with objects, and social play. Crocodiles, the new study reveals, enjoy all three.

Crocs most frequently engage in object-play. The most common observation of crocodile play (outside the most recent study) is that of the reptiles playing with balls, ceramic bits, remnants of previously devoured prey, and other floating debris.

The animals have also been observed (in and out of the study) enjoying locomotor play: repeatedly sliding down river banks and surfing waves and currents.

Finally, crocodiles were observed frequently playing with their family members and friends, both in captivity and in the wild. Piggy back rides were the most common form of social play.

Crocs have even been seen playing with other animals, including river otters. And in rare cases, injured alligators rescued by humans have formed playful bonds with their caretakers. In one report, a man who rescued a croc that had been shot in the head ended up playing with the animal every day for twenty years.

"The croc would swim with his human friend, try to startle him by suddenly pretending to attack him or by sneaking up on him from behind, and accept being caressed, hugged, rotated in the water, and kissed on the snout," study author Vladimir Dinets explained in a press release.

"Hundreds of thousands of crocodilians are now kept in captivity in zoos, commercial farms, and breeding centers set up for endangered species. Providing them with toys and other opportunities for play makes them happier and healthier," Dinets added.

Because play is a sign of intelligence, Dinets and his colleagues say studying playful behavior in animals not necessarily associated with recreation can help shed light on the definition and development of intelligence.

The new research was published this week in the journal Animal Behavior and Cognition.